Cathode ray tube



H. W. PAEHR CTHODE RAY TUBE Sept. 29, 1942.

lf'iled Jan. 26. 1940 H II Patented Sept. 29, 1942 CATHODE RAY TUBE HansWerner Paehr, Berlin-Charlottenburg, Germany; vested in the AlienProperty Custodian Application J anuary 26, 1940, Serial N0. 315.635 InGermany February 9, 1939 Claims.

The invention relates to cathode ray tubes and in particular to theelectron gun arranged Whin a cathode ray taube.

It is an object of the invention to provide a cathoole ray tube having acathode ray of small divergence and small cross section in the plane ofthe screen. It is a further object to utilize an electron gun produclngsuch a ray in a taube having a comparatively large screen and a shortdistance between the cathode and the screen so that a wide angle ofcleflection must be used to scan the entire screen surface.

It is a further object to improve the electron gun systems hitherto inuse with respect 130 control sensitivity and sharpness of the Spot.Other objects of the invention will be apparent from the followingdescription in connection With the drawing representing in Fig. 1 alongitudinal section through the neck portlon 0f the cathode ray tubeand in Fig. 2 a side view 01 a complete taube.

The most simple and in first approximation also true explanation of theelectron optical arrangement 0f a cathode ray tu'be is that the emittingsurface of the cathode is reproduced upon the screen of the tube bymeans of an electron lens. It has however been ascertained that thesharpest spot is obtained no1: as an image of the cathode surface lautas an image of a cross section of the cathocl ray immediately in frontof the cathode ab a point where the cathode ray has its smallestdiameter. 'Ihis cross section is usually called the cross-over point.

Up to the present filme the development 0f electron gun systems has beenin the directiou to increase as far as possible the ratio between theemitting area of the cathocle and the area 0f the cross section of thecathocle ray in the cross-over point in order to reduce the specificemission load of the cathode surface and to obtain in spite thereof ahigh int-ensity in the cathocle ray. This development was in thedirection to large angles of divergency.

According to the invention it has been founcl thao it is not preferableto use a ray With a large angle cf divergency but that it is better touse a cathode ray With a divergency as sma1l as possible.

The invention is of particular importance for television receiving tubeswith intensity control by means of a variation of the potential of aWehnelt cylinder and furthermore in connection with tubes having a largeangle of defiection of 45 and more towards each side from the aXis ofthe tube. Such tubes have been suggested in order t0 obtain a shorttotal length of the tube. Tubes of this type make use of inhomogeneousdeflecing fields in order to obtain the necessary deflection upon ascreen With a large radius of curvature. The consequence is that theluminescent spot is not sharply focused near the edges of the screen. Ithas however been found that this drawback can be overcome if a verynarrow athode ray is used having a divergency which is so small that theinhomogenity -0f the deflecting field can be neglected within the spacebetween the boundaries of the cathode ray.

Experiments about the requirements for the construction of a suitableelectron gun have shown that an improvement of the directional efiectcan be obtained -by increasing the ratio between the cut 01T voltage atthe Wehnelt cylinder and the anode volta-ge. An increase of this ratioresults however in a loss of control sensitivity. The invention points away to improve the directional efiect and 170 obtain a cathode raybundle 0f small divergency without a simultaneous loss of controlsensitivity.

Aecording to the invention the Wehnelt cylinder includes a front portionwhich has at least partly the form of a cone o1 funnel opening towardsthe anode of the taube. It has been found that the inclination of thesides of the cone is very essential for obtaining the desired eflect.According to the invention the opening of the cone has an angle ofapproximately In Figs. 1 and 2 the cathode ray taube 6 has a neckportion l of cylindrical cross section. The neck portion has a foot lsupporting a cathode 2 and a Wehnelt cylinder 3. The cathode is of thehairpin type carrying a small ba1l at the point of the bend. The Wehneltcylinder consists of a cylindrical portion surrounding the cathode and aconical portion immediately in front of the cathode. The conical portionhas a central aperture and opens towards the screen of the tube. Theanode 4 has the form of a Wall coating ending shortly before theWehnelt; cylinder. The concentrating fields is produced by a coi1 5 onthe outsicle of the neck of the taube. systems are not represented inthe drawing.

A system as described in connection with Fig. 1 produces a cathode rayof unexpected sma1l divergency and has the advantage that the divergencyremains practically constant when the current is increased whilehitherto an increase of the current has always resulted in an increaseof the angle of divergency. The gun system of the invention producestherefore a spot of more uniform diameter in intensity controlled tubes.

The deflecting The experiments have shown that an angle of the cone of90 and less, or of 160 and more has given much lass favourable results.

The opening Within the Wehnelt cylinder shall be as small as possible; adiameter of 1 mm. Will however be the lower limit for practical reasons.The height of the cone in the direction of ehe tube axis shall be atleast three or better four times as lal'ge as the diameter of the sma1laperture f the Wehnelt cylinder. Instead of a conical Wall portion it ispossible to use a heavy plate having a conical depression on the sidetowards the anode. The Wehnelt cylinder shown in the drawing isproduced, for example, by pressing or stamping.

It is particularly preferable 130 use this form of Wehnelt cylinder incombination with a cylindrical anode of large opening. This is contraryto usual systems in which the anode has the form of a flat disc with asmall aperture. The inner diameter of the anode cylinder shall be atleast larger than the outer diameter of the Wehnelt orcontrol electrode.

What I claim is:

1. A cathode-ray device adapted to provide an angle of ray deflection ofthe oraler of 45 or higher comprising a cathocle, a conical controlelectrode apertured at its apex and positioned with said apex acljacentsaid cathoole, and an anode comprising a hollow cylinder adjacent saidconical electrode and unobstructed therefrom, the diameter of saidcylindrical anode being greater than the outer diameter of said conicalelectrode whereby to provlde a cathode ray bundle of small divergencywithout simultaneous loss of control sensitivity.

2. A cathode-ray device adapted to provide an angle of ray deflection ofthe order of 45 or higher comprising a cathode, a control electrodehaving a cylindrical portion surrounding said cathode and a conicalportion apertured at its apex and positioned. With said apex adjacentsaid cathode, and an anode comprising a hollow cylinder adjacent saidconical electrode and unobstructed therefrom, the diameter of said cy1indrical anode being greater than the outer diameter of said conicalelectrode whereby to provide a cathode ray bundle 0f small divergencyWithout simultaneous loss of control sensitivity.

3. A cathode-ray device adapted to provide an angle of ray deflection ofthe order of 45 or higher comprising a cathode, a conical controlelectrode having an apex angle between and 160 and being apertured atits apex and positioned with said apex adjacent said cathode,

and an anode comprising an unobstrucced hollow cylinder, the diameter ofsaid cylindrical anode being greater than the outer diameter of saidconical electrode whereby to provide a oathode ray bundle of smalldivergency without simultaneous loss of control sensitivity.

4. A cathode-ray device adapted to provide an angle of ray deflection ofthe Order of 45 or higher comprising a cathode, a conical controlelectrode having an apex angle of approximately and being apertured atits apex and positioned With said apex adjacent said cathode, and ananode comprising an unobstructed hollow cylinder, the diameter of saidcylindrical anode being greater than the unter diameter of said conicalelectrode whereby to provide a cathode ray bundle of small divergencywithout simultaneous loss of control sensitivity.

5. A cathode-ray device adaptecl to provide an angle of ray deflectionof the order of 45 o1 higher comprising a cathode, a conical controlelectrode having an apex angle cf approximately 120 and being aperturedat its apex and positioned with said apex adjacent said cathode, thedepth of said conical electrode being ab least th.ree times the diameterof the aperture of said conical electrode, and. an anode comprising anunobstructed hollow cylinder, the diameter of said cylindrical anodebeing greater than the outer diameter 01 said conical electrode Wherebyto provide a cathode ray bundle of small divergency without simultaneousloss of contro1 sensitivity.

HANS WERNER PAEHR.

